Metallurgical process and steels manufactured by same



nited States Patent Steel Company, Hammond, Ind., a corporation of Delaware No Drawing. Filed June 29, 1959, Ser. 'No. 823,322 9 Claims. ((1148-12) This invention relates to a method of cold finishing steel bars and rods.

It is an object of this invention to produce steel bars and rods having new and improved physical and mechanical properties and it is a related object to provide a method applicable to the cold finishing of steel bars and rods to improve the physical and mechanical'proper-ties thereof.

More specifically, it is an object of this invention to provide a method applicable to bars and rods formed of steels which strain harden and harden by some mode of precipitation during working at elevated temperatures to improve mechanicaland physical properties of the steel bars and rods during the cold finishing thereof.

As used herein, the term steel is meant to refer to bars and rods formed of steel of the non-austenitic type having a pearlitic structure in a matrix of free ferrite or to steels which strain harden and harden by some mode of precipitation when worked at elevated temperatures in excess of 400 F. and below the lower critical temperature of the steel. It is not intended to include sheets or tubing formed of steel.

As used herein, the term advance through a die to achieve reduction in cross-sectional is meant to include the process of either drawing or extrusion, as distinguished from rolling.

This invention is addressed to the process wherein physical and mechanical properties of different values are developed in the steel when the steel is normalized prior to advancement of the steel through a die to eifect reduction in cross-sectional area while the steel'is at'a temperature within the range of 250 F. to the lower critical temperature for the steel composition and preferably while the steel is at a temperature within the range of 400-950 F. The combination of steps which comprises a normalizing in advance of the elevated temperature reduction step overcomes the tremendous variation in properties of hot rolled steels and makes available cold finished steel bars, rods and the like having greater uniformity of properties from bar to bar between heats. Another significant improvement of the process of this invention resides in the production of steels having improved ductility while maintaining the high strength levels of the steel. Still further, it has been found that bars reduced while at elevated temperature after being processed to provide a normalized structure are markedly improved in their maehinability characteristics.

The system described and claimed herein, which makes use of a normalizing step in advance of the passage of the steel through a die to effect reduction in cross-sectional area while the steel is at an elevated temperature within the range described, is somewhat independent of the amount of reduction that is taken in the steel during the reduction step. It is somewhat dependent on the chemistry of the steel, the temperature of the steel advanced through the die to achieve reduction in crosssectional area and the means for cooling the steel to ambient temperature. Differences in temperature, chemistry and methods of cooling will tend to maximize certain of the properties of the steel, as will hereinafter appear.

Steels capable of use in the practice of this invention may be characterized as hot rolled steels and by the ability of the steel to strain harden and harden by some ice mode of precipitation when worked at elevated temperature within the range described. Thus steels which can be employed in the practice of this invention include the non-austenitic steels which have a pearlitic structure in a matrix of free ferrite.

As used herein, the term normalizing is meant to define the process in which a ferrous metal or alloy is heated to a suitable temperature above the transformation range and is subsequently cooled in still air at room temperature. In accordance with the practice of this invention, the steel bars or rods may be advanced to the die for the described elevated temperature reduction step when the steel has air cooled to the temperature desired for the elevated temperature reduction step, such as at a temperature above 250 F. to below the critical temperature for the steel composition. Instead, the steel can be air cooled from normalizing temperature to ambient temperature and thereafter reheated to an elevated temperature for elevated temperature reduction while at a temperature within the range of 250 F. to the lower critical temperature for the steel composition.

The concepts of this invention will hereinafter be illustrated by reference to the processing of four steels which were taken as representative'of the classes of steels which may be employed. These representative steels will hereinafter be referred to as C1018, C1144, C1080 and 4140. The following is a ladle analysis of these steels in which the major ingredients, other than iron, are set forth:

endows Gown- 00 The proceduresfor processing the steels and the conditions for testing the steels in the development of the data set forth in the following tables will hereinafter briefly be described along with some of the terms which are employed as used in the specification and claims.

The C1018 steel was used as inch round, the C1144 steel was used as inch round, the C1080 steel was used as inch round, and the 4140 steel was used as inch round. The hot rolled steel bars, as received, were descaled by pickling in sulphuric acid and limed to prevent rusting. Lime has the advantage of preventing the formation of a tight scale in a normal furnace atmos-.

ing the concepts of this invention.

The normalized bar stock was reheated for drawing in a gas-fired furnace and a suitable drawing lubricant was .applied to the metal surface prior to drawing. For normalizing, the steels were heated to the following temperature C10l8l650 F. 01144-1650 F. C1080-4650" F. 44140-1600 F.

The cold reduction stepswere taken by advancing the steel through a draw die to effect reduction in crosssectional area while the steel was at room temperature. Reduction at elevated temperature was carried out by advancing the steel bars through a draw die to effect reduction in cross-sectional area while the steel was at a desired elevated temperature.

Definitions.Percent reduction is meant to relate to the true reduction as represented by the formula -5 X 100 percent reduction wherein D is the original hot rolled diameter of the steel and D is the final diameter of the steel.

Proportional limits is intended to correspond to the point on the stress-strain curve where the greatest stress that the material is capable of sustaining without deviation from the law of proportionality of stress to strain occurs (Hookes Law). This point is of particular portance in steel and, in practically every instance, it is measurably increased to exceptionally high values when steels, such as steels of the non-austenitic type, are processed .by drawing or extrusion while the steel is at an elevated temperature within the range described.

Warpage factor is directly related to residual stress. The warpage value is an indication of the concentration and character of the longitudinal stresses present in steel. The residual stress is obtained by means of a warpage test wherein the length of the test piece is determined as being five times the diameter plus 2 inches. The test pieces are slotted through a diameter for a distance five times the diameter. The length of the slot is recorded and the maximum diameter perpendicular to the slot is also recorded. The dilferences between the diameter before slotting and after slotting represent the flare caused by the presence of residual stresses. The flare is considered positive, indicating a preponderance of tensile stresses in the bar, if the bar expands on slotting. The fiare is considered negative, indicating a preponderance f0 compressive stresses in the bar, if the ends move towards the cut which is made through the diameter. The warpage values determined for evaluation are calculated on the following equation:

(Ls), x100 Warpage Factor where D =the original diameter of the bar before cutting the slot D =the diameter differential before and after cutting the slot (flare) L =length of slot reading of the drawing load minus chain drag was converted to pounds pull by multiplying by a factor of 22.9. This factor was obtained from the slope of a calibration curve of Waterbury-Farrell hydraulic pressure versus Tinius-Olson tensile testing machine pound pull.

Izod impact represents the impact in foot pounds obtained by averaging the impact results from three equidistant 45 degree notches (0.13" deep) at F. on a 0.45 diameter round X 4 /2 long specimen.

Hardness, represented by a Diamond Pyramid Number (DPN), was measured on a Gries Reflex testing machine employing a 136 Diamond Pyramid at a 50 kg. load.

By the term cold reduction or cold drawing (CD) is meant the taking of a reduction in a cross-section of the steel bar at about ambient temperature. It will be understood that the term cold drawing will include advancement of the steel through a die to effect reduction in cross-sectional area at a temperature slightly above ambient temperature on up to about 200 F.

The term elevated temperature drawing (ETD) is meant to define the taking of a reduction in the crosssection of the steel by advancement of the steel through a die while at a temperature within the range of 250 F. to the lower critical temperature for the steel composition (1100-1400 F.).

The data in the following Tables I to VIII has been arbitrarily selected to present the better of the listed properties secured by elevated temperature reduction in the steels drawn at elevated temperature without first normalizing and after normalizing. The data sets forth the mechanical and physical properties of the hot rolled steels, steels drawn to effect an equivalent reduction at ambient temperature (CD), the same steels drawn at an elevated temperature (ETD) and the same steels which were normalized prior to cold drawing or drawing at elevated temperature. The last represents the improved practice of this invention. In the development of the data, steels of the same composition were employed in the various tests. The amount of reduction was held as nearly the same as possible and the reduction at elevated temperature (ETD) was carried out at various temperatures within the range of 250 F. to the lower critical temperature for the steel composition.

It will be understood that the combination of steps which combines a normalizing step in advance of an elevated temperature reduction step permits the development of properties in steel which'heretofore have been incapable of formation in the steel by conventional methods of cold finishing or even, in some instances, by the previously described method of drawing at elevated temperature.

In the following tables,

A-=Air quenched O=Oil quenched W=Water quenched TABLE I C1018 steel drawn to take 17.2% reduction air cooled to room temperature after drawing Tensile Yield Elonga- Red. of Warpage factor Izod im- Hardness, Process strength, strength, tion 1.4, area, perrange pact 70 F., DIN, MR

p.s.i. p.s.i. percent cent ft.lbs.

68, 375 46, 875 36.0 67. 9 021 87.0 151 Cold drawn (A) 103, 000 103,000 13.0 52. 1 003 23. 0 223 (A) 1 116, 250 1 116, 250 1 27. 5 3 58. 6 001 to 184 3 11. 7 266 Normalized hot roll 69, 000 51, 000 34. 0 68. 6 021 (83. 3) 164 Normalized cold drawn (A 105, 000 105,000 13.0 52. 1 034 9. 3 213 Normalized plus ETD (A) 4 122, 500 4 122, 500 5 27. 0 B 59.2. 006 to 218 18.3 211 See Table II for footnotes.

Norm-Number in parentheses denotes averaged value for fibrous fracture-not a clean break test.

. TABLE I1 C1018A steel drawn to take 15.7% reduction Tensile Yield Elonga- Bed. of Warpage factor Izod lm- Hardness, Process strength, strength, tion 1.4", area, perrange pact 70 F., DPN, MR

p.s.i. p.s.i. percent cent ft.lbs.

68, 375 46, 875 36. 67. 0 021 87. 0 151 92, 000 92, 000 14. 59. 5 075 15.0 207 5 110, 000 99, 000 Y 7 28. 0 7 62. 7 81 to 309 B 21. 3 249 69, 51, 000 84; 0 68. 6 021 (83. 3) 164 Normalized cold drawn 102, 500 102, 500 15.0 52. 5 040 15. 7 213 Normalized plus ETD (w).- 8 108, 500 B 107, 500 9 30. o 1 61. 0 023 to 361 62.7 241 l Drawn at 545 F. 5 Drawn at 710 F. 1 Drawn at 1,000 F. 7 Drawn at 1,0l0 F. a Drawn at 450 F. B Drawn at F. 4 Drawn at 500 F. Drawn at 980 F. l Drawn at 915 F. Drawn at 910 F. Norm-Number in parentheses denotes averaged value for fibrous fracture-not a clean break test.

TABLE III C1080 steel drawn to take 15.7% reduction air cooled after drawing Tensile Yield Elonga- Red. of Warpage factor Izod im- Hardness, Process strength, strength, '131011 1.4", area, perrange pact F., DPN, MR

p.s.1. p.s.i. percent cen ft.lbs.

144, 500 76, 000 12. 5 17. 0 025 4. 7 296 170, 500 148, 500 5. 0 14. 5 077 3. 3 324 1 189, 000 1 186, 000 3 10.0 3 12.4 077 5 5.0 378 154, 750 86,250 11.0 23. 6 036 5. 7 296 173, 500 127, 500 6. 0 18. 6 101 5.0 343 Normalized plus ETD (11)---" 4 190,000 4 183, 500 7 9.5 B 23.1 047 to 062 5 7. 0 356 See Table IV for footnotes.

' TABLE IV C1080 steel drawn to take 15 .7% reduction quenched in oil or water after drawing Tensile Yield Elonga- Bed. of Warpage factor Izod im- Hardness, Process strength, strength, tion 1.4", area, per range pact 70 F., DPN, MR

psi. p.s.i. percent cent ft.lbs.

Hot roll 144, 500 76, 000 12. 5 17.0 025 4. 7 296 Cold drawn (W)- 163, 5 137, 500 4. 5 8. 8 077 3. 0 324 E D (W) 186, 750 5 183, 500 7 8.0 11.0 069 to 046 4. 0 371 Normalized hot roll 154, 750 86, 250 11. 0 23. 6 036 5. 7 296 Normalized cold drawn (0) 168, 126, 250 6. 4 16.0 101 6.0 336 Normalized plus ETD (O) 5 193, 500 5 186, 000 9 10.7 17.6 023 to 093 9 7. 7 356 l Drawn at 610 F. Drawn at 620 F. 1 Drawn at 940 F. 1 Drawn at 955 F. 3 Drawn at 815 F. Drawn at 650 F. 4 Drawn at 630 F. Drawn at 950 F. 5 Drawn at 650 F.

TABLE V 4.140 steel drawn to take 19.9% reduction air cooled after drawing Tensile Yield Elonga- Red. of Warpage factor Izod im- Hardness, Process strength, strength, tion 1.4, area, perrange pact 70 F., DPN, MR

p.s.i. p.s.i. percent cent it.-1bs.

140, 000 105, 750 15. 0 42. 8 004 9. 0 307 165, 000 162, 000 9. 0 44. 0 083 4. 3 336 l 189, 500 1 189, 000 7 l6. 5 2 49. 4 211 to 064 7 15.0 394 157, 000 101, 250 16. 5 39. 7 083 12. 0 353 Normalized cold drawn (A)- 180, 000 174, 000 6. 0 26. 9 096 7. 3 350 Normalized plus ETD 11)..- a 201,000 1 198,000 1 21.0 1 50. 5 +.141 to 001. 1 30. a 402 See Table V1 for footnotes. I f 1 TABLE VI 4140 steel drawn to take 19.9% reduction quenched in oil after drawing Tensile Yield Elonga- Red. of Warpage factor Izod im- Hardness, Process strength, strength, tion 1.4", area, perrange pact 70 F DPN, MR

p.s.i. p.s.i. percent cent. 11.-lbs.

H roll 140, 000 104, 750 15.0 42. 8 004 9. 0 307 Gold drawn (0)..... 160, 000 153,000 9.0 48. 9 159 6. 0 330 ETD (O) 5 195, 000 5 191, 250 5 17.5 B 45. 7 262 to +.026 12.0 402 Normalized hot r011 157, 000 101, 250 16. 5 39. 7 083 12.0 353 Normalized cold drawn (0). 198,000 193, 2.5 9.4 026 V 4.0 371 Normalized plus ETD (O) 7 195, 500 7 193,000 9 20.0 B 52. 9 to 006 9 18.0 411 0 Drawn at 960 F. 7 Drawn at 520 F.

1 8 Drawn at 970 F.

Drawn at 910 F.

perature without previously normalizing the steel.

The foregoing data will clearly illustrate the improvements, as for example, in tensile strengths and yield strengths, which are capable of being secured by the com bination of steps which make use of a normalizing step in advance of passage of the normalized steel through a die to etfect reduction in cross-sectional area while the steel is at an elevated temperature. The improvements are shown by comparison with equivalent reductions of the same steels at room temperature after the equivalent normalizing step or by comparison with the advancement of the same steels through a die to effect reduction in cross-sectional area While the steel is at an equivalent elevated temperature, but without previously normalizing the steel.

While not shown in the tables, one of the characteristics of the steel most noticeably increased by the combination of normalizing and elevated temperature reduction comprises the machinability characteristic of the steel. Normalizing and drawing at elevated temperature markedly improves the machinability of the steel by comparison with steels drawn at an equivalent elevated tem- In addition, it will be apparent that the strength levels are maintained while improvements are secured in the duetility of the steels normalized and drawn at elevated temperature by comparison with the values secured by elevated temperature reduction alone. A property not apparent on its face is the marked improvement which is secured in the uniformity of properties from bar to bar between heats.

8 While it will be apparent that certain of these properties are maximized at different temperatures, it becomes possible to achieve improvements in selected physical and mechanical properties by selection of the chemistry of the steel, the amount of draft, and by proper selection of the temperature of the steel during advancement through the die, when the reduction step at elevated temperature is combined with prior heat treatment to normalize the steel. Thus, chemistry, reduction, and temperature play an important part in the specific improvements of certain of the properties of the steel to maximize one property by comparison with another and to produce steels in which the property maximized may be considerably better than has heretofore been made available by other comparable cold-finishing, heat-treating, or strain-relieving systems.

The data which will be presented in the following tables illustrates the improvements in properties of the steels TABLE VII C1018 steel normalized at 1650" F., 17.2% reduction by by drawing, air cooled after drawing Tensile Yield Elonga- Bed. of Warpage Izod impact Hardness, Temp. of draw, F. strength, strength, tion, area, factor F., DPN,

p.s.i. p.s.i. percent percent tt.-lbs. MR

N urn-N umber in parentheses denotes averaged values for a fibrous fracture-not a clean break test.

TABLE VIH C1018 steel normalized at 1650 F., 1 7 .2 reduction by drawmg, water quenched after drawmg Tensile Yield Elonga- Bed. of Warpage Izod impact Hardness, Temp. of draw, F. strength, strength, tlon, area, factor 70 F., DPN,

p.s.i. p.s.i. percent; percent ft.-lbs. MR

Hot roll 68, 375 46, 875 36. 0 67. 9 021 87. 0 151 Normalized hot ro11 69, 000 51, 000 34.0 68. 6 021 (83. 3) 164 200 102, 500 102, 500 15.0 52. 5 040 15. 7 213 395. 106, 250 106, 000 13. 0 50. 9 080 13. 7 241 530- 108, 500 108, 000 12. 5 49. 7 103 3.0 234 600- 108, 500 107, 500 11.0 52. 1 092 3.0 241 685. 102, 500 102, 500 14. 5 63.7 023 6. 3 238 800. 97, 500 92, 500 20.0 68. 8 023 23.0 223 910. 87, 000 75, 000 25. 5 59.9 103 62. 7 980 84, 500 69, 500 30. 0 p 61.0 361 (89. 7) 195 Norm-Numbers in parentheses denote averaged values for a fibrous fracture-not a clean break test.

TABLE XIII Tensile Yield Elonga- Bed. of Warpage Izod impact Hardness, Temp. of draw, F. strength, strength, tion, area, factor 70 F., DPN,

p.s.i. p.s.1. percent percent ft.-1bs. MR

Hot roll .1 140,000 105, 750 15. 42. 8 004 9. 0 307 Hot rolled normalized. 157, 000 101, 250 16. 39. 7 083 12.0 353 270. 180, 000 174, 000 6. 0 26. 9 096 7. 3 350 420. 170, 000 168, 750 9. 5 38. 9 121 12.0 371 5101 181, 500 180, 750 11.0 42. 8 089 11. 7 378 590 201,000 198,000 14. 5 31. 7 141 7. 7 402 710. 183, 000 183, 000 13. 0 47. 7 121 11. 0 394 810... 162, 500 156, 000 16. 0 51. 3 121 16. 0 350 905--- 149, 500 136, 500 18.0 52. 1 006 30. 3 324 960 138, 000 117, 750 21. 0 50. 5 001 (50. 0) 301 Nora-Number in parentheses denotes averaged values for fibrous tracture-not a clean break test.

TABLE XIV 4140 steel normalized at 1600 F., 19.9% reduction by drawing, ozl quenched after drawing Tensile Yield Elonga- Bed. of Warpage Izod impact Hardness, Temp. of draw, F. strength, strength, tion, area, factor 70 F., DPN,

p.s.i. p.s.i. percent percent ft.-lbs.

Hot roll 140, 000 105, 7 50 15.0 42. 8 004 9.0 307 Hot rolled normalized. 157,000 101, 250 10. 5 39. 7 083 12.0 350 280 198, 000 193, 500 2. 5 9. 4 026 4. 0 371 105, 000 188, 750 5. 0 l4. 0 057 8. 7 378 195, 500 193, 000 8. 0 32. 1 045 9. 0 411 183, 250 182, 000 12. 5 44. 9 128 9. 0 386 185, 000 183, 500 13.0 48. 5 006 9. 3 394 172, 000 162, 000 14. 6 46. 1 140 12. 7 371 153,000 135, 000 16. 5 46. 1 026 18.0 330 133, 500 113, 500 20. 0 52. 9 006 (54.0) 291 Norm-In parentheses denotes averaged values for fibrous fracture-not a clean break test.

It will be apparent from the foregoing that steels having physical and mechanical properties differing in many respects from steels of the same composition can be secured by the combination of steps which makes use of a normalizing step in advance of an elevated temperature reduction step. It will be seen from the foregoing that a relationship exists between chemistry, percent reduction and temperature which greatly distinguishes the described process of normalizing and elevated temperature reduction from that of cold drawing or from that of normalizing and cold drawing. It will be seen from Tables IX to XVI that the same type of relationship also exists in the improvements in yield strength, elongation, reduction or area and hardness and to material quenched in oil or water after drawing, as well as that which is air cooled.

In general, pull load and residual stress decrease while the izod impact improves as the temperature of the steel in the reduction step increases. From the standpoint of strength values, such as tensile strength, yield strength and the like, maximum improvements have been found to take place in all of the steels processed in accordance with the practice of this invention when the elevated temperature reduction step is taken while the steel is within the range of 400-950 F., and that for overall improvements in properties it is desirable to make use of a temperature within the range of about 250 F. to the lower critical temperature for the steel composition for the steel reduced in the elevated temperature reduction step.

From the foregoing, it will be apparent that I have provided a process to improve physical and mechanical properties of steel bars and rods in a cold finishing operation. While the discussion may have, referred chiefly to the improvements as represented by tensile and yield strengths, other of the properties of the steel are also affected by the combination of steps embodying normalizing in advance of elevated temperature reduction.

It will be understood that numerous changes may be made in the details of operation and conditions of oper-ation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. The metallurgical process for the improvement of physical and mechanical properties of steel bars and rods of the non-austenitic type having a pearlitic structure in a matrix of free ferrite, consisting of the following combination of steps in the order specified of normalizing the steel and advancing the steel bars and rods through a draw die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 250 F. to the lower critical temperature for the steel composition.

2. The metallurgical process for the improvement of physical and mechanical properties of steel bars and rods which strain hardens and hardcns by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, consisting of the following combination of steps in the order specified of normalizing the steel and subsequently advancing the steel bars and rods through a draw die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 250 F. to the lower critical temperature for the steel composition.

3. The metallurgical process for the improvement of physical and mechanical properties of steel bars and rods which strain hardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, consisting of the following combination of steps in the order specified of normalizing the steel bars and rods and subsequently advancing the normalized steel through a draw die to eflect reduction in cross-sectional area while the steel is at a temperature within the range of 250 F. to the lower critical temperature for the steel composition, and air cooling the steel after advancement through the die at elevated temperature.

4. The metallurgical process tor the improvement of physical and mechanical properties of steel bars and rods which strain hardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, consisting of the following combination of steps in the order specified of normalizing the steel bars and rods and subsequently advancing the normalized steel through a draw die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 250 F. to the lower critical temperature for the steel composition, and quenching the steel rapidly to cool the steel after advancement through the die at elevated temperature.

5. The metallurgical process for the mprovement of physical and mechanical properties of steel bars and rods which strain harden and harden by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition consisting of the following combination of steps in the order specified of normalizing the steel by heating the steel to austenitizing temperature followed by slow cooling to a temperature a range of 400 F. to the lower critical temperature for the steel composition to normalize the steel, advancing the normalized steel through a draw die to elfect reduction in cross-sectional area while the steel is at the elevated temperature within the range of 400 F. to the lower critical temperature for the steel composition.

6. The metallurgical process for the improvement of physical and mechanical properties of steel bars and rods which strain harden and harden by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition consisting of the following combination of steps in the order specified of normalizing the steel by heating the steel to austenitizing temperature followed by slow cooling to a temperature within a range of 400 F. to the lower critical temperature for the steel composition to normalize the steel, advancing the normalized steel through an extrusion die to effect reduction in cross-sectional area while the steel is at the elevated temperature within the range of 400 F. to the lower critical temperature for the steel composition.

7. The metallurgical process for the improvement of physical and mechanical proper-ties of steel bars and rods which strain harden and harden by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition consisting of the following combination of steps in the order specified of normalizing the steel by heating the steel to austenitizing temperature, air cooling the steel to ambient temperature, reheating the steel to a temperature within the range of 400 F. to the lower critical temperature for the steel composition tonormalize the steel, and advancing the normalized steel through a draw die to effect reduction in cross-sectional area in 'an amount greater than 15.7 percent while the steel is at an elevated temperature within a range of 400 F. to the lower critical temperature for the steel composition.

8. The metallurgical process for the improvement of physical and mechanical proper-ties of steel bars and rods which strain harden and harden by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition consisting of the following combination of steps in the order specified of heating the steel to austenitizling temperature, air cooling the steel to ambient temperature to normalize the steel, reheating the steel to a temperature within the range of 400 F. to the lower critical temperature for the steel composition, and advancing the normalized steel through an extrusion die to effect reduction in cross-sectional area while the steel is at an elevated temperature within the range of 400 F. to the lower critical temperature for the steel composition.

9. A steel product having improved physical and mechanical properties produced by the method of claim 2.

References Cited in the file of this patent Iron & Steel Eng., by Dunn, July 1946, pages 51-57 and 77.

Transactions of the A.S.M., vol. 45, pages 333-343, 1953.

Mechanical Engineers Handbook, page 588, 4th edit-ion. 

1. THE METALLURGICAL PROCESS FOR THE IMPROVEMENT OF PHYSICAL AND MECHANICAL PROPERTIES OF STEEL BARS AND RODS OF THE NON-AUSTENITIC TYPE HAVING A PEARLITIC STRUCTURE IN A MATRIX OF FREE FERRITE, CONSISTING OF THE FOLLOWING COMBINATION OF STEPS IN THE ORDER SPECIFIED OF NORMALIZING THE STEEL AND ADVANCING THE STEEL BARS AND RODS THROUGH A DRAW DIE TO EFFECT REDUCTION IN CROSS-SECTIONAL AREA WHILE THE STEEL IS AT A TEMPERATURE WITHIN THE RANGE OF 250* F. TO THE LOWER CRITICAL TEMPERATURE FOR THE STEEL COMPOSITION. 